Microsoft Word - AG_56.FT1.2014_Casadio_ONLINE_.doc


ANNALS OF GEOPHYSICS, 56, Fast Track 1, 2013; 10.4401/ag-6535 
 

The Atmospheric Composition Validation 
and Evolution Workshop (ACVE2013) - 

Recommendations 

Stefano Casadio1,2, Angelika Dehn1, Thorsten Fehr1, Bojan R. Bojkov1 
1ESA-ESRIN, Frascati, Italy 

2SERCO-IDEAS, c/o ESA-ESRIN, Frascati, Italy 
 
 

I. INTRODUCTION 

uring the last 18 years, the European 
Space Agency (ESA) has provided the 
scientific community with a large 

amount of valuable atmospheric composition 
data produced by a series of instruments, start-
ing with GOME [Burrows et al. 1999], on-
board the ERS-2 satellite (1995-2011), and fol-
lowed by the GOMOS [Kyrola et al. 2004], 
MIPAS [Fischer et al. 2008], and the 
SCIAMACHY [Bovensmann et al. 1999], all 
flying on-board the Envisat satellite (2002-
2012). These observations will be continued by 
the Sentinel-5 Precursor, Sentinel-4 and Senti-
nel-5 and extended the EarthCARE and ADM 
missions for aerosols and clouds. 
The quality assessment of atmospheric param-
eters retrieved from ESA satellite instruments 
has been, and continues to be, performed 
through the joint effort of the scientific com-
munity and ESA. The calibration and valida-
tion of atmospheric products (Cal/Val), for 
both measured radiances (Level-1) and re-
trieved atmospheric parameters (Level-2), is a 
continuous process that starts well before the 
launch of the satellite, and continues after the 
decommissioning of the platform. In this con-
text, a large number of activities are promoted 
by ESA with the support of the Mission Ad-
visory Groups (MAGs) during the pre-launch 
phases of the missions, and the Quality Work-
ing Groups (QWGs) during the post-launch 
phases of the missions. 
The knowledge gained by ESA’s Cal/Val ac-
tivities is made publicly accessible to the at-

mospheric community through scientific and 
technical publications, as well as through a se-
ries of dedicated workshops and conferences 
sponsored by ESA. The series of workshops 
dedicated to the Cal/Val of atmospheric pro-
ducts started with the “Pre-launch Workshop 
on the Atmospheric Chemistry Validation of 
Envisat (ACVE)” in 2001 [ACVE 2001], fol-
lowed by the “ENVISAT Validation Work-
shop” in 2002 [Envisat Validation 2002]. Two 
years afterwards, in 2004, ESA organised the 
second “Atmospheric Chemistry Validation of 
Envisat (ACVE– 2) [ACVE-2 2004], followed 
by the third “Atmospheric Chemistry Valida-
tion of Envisat (ACVE– 3)” in 2006 [ACVE-3 
2006]. After each Cal/Val workshop, ESA was 
provided by the participants with a compre-
hensive list of comments, suggestions and rec-
ommendations, critical for the successful oper-
ations of ESA missions [Snoeij et al. 2004, 
Snoeij et al. 2007]. Cal/Val recommendations 
are also provided to ESA in occasion of confer-
ences with broader scopes. An important and 
recent example is represented by the ATMOS 
conference, organised by ESA and held in 
Bruges (Belgium) on 18-22 June 2012 
[ATMOS2012]. In particular, the recommenda-
tions regarding spectroscopy studies, the re-
processing capabilities, and the mainte-
nance/enhancement of the validation infra-
structure are important contributions to the 
Cal/Val activities. Apart from specific recom-
mendations to the ERS-2 and ENVISAT mis-
sions, a number of general considerations were 
raised during past ACVEs. The most relevant 
recommendations are summarised below: 

D



ANNALS OF GEOPHYSICS, 56, Fast Track 1, 2013; 10.4401/ag-6535 
 

• Improve communication between Level-1 
and Level-2 development teams; 

• Provide information about data processor 
changes to users in a transparent way; 

• Document and announce well in advance 
(whenever possible) all changes in proces-
sors, algorithms and initialisation files; 

• Product user manuals should be well writ-
ten, including detailed format and content 
description; 

• Auxiliary files should be available and doc-
umented; 

• A well-defined validation reference set 
should be processed completely after each 
processor upgrade and analysed before re-
processing the complete mission. 

These recommendations have all been fulfilled 
by ESA during the ENVISAT mission lifetime, 
and are now guidelines for future missions. 
In March 2013, ESA organised the “Atmos-
pheric Composition Validation and Evolution 
(ACVE2013)” [ACVE 2013] workshop with a 
much wider scope including: 1) Future Mis-
sions and Evolution [e.g., Castelli et al. 2013]; 
2) Instrument Calibration and Level-1 pro-
ducts; 3) Methods and Instruments for Valida-
tion [e.g., Garcia et al. 2013]; 4) Algorithms and 
Products [e.g., Raspollini et al. 2013]; 5) Ozone 
[e.g., Waymark et al. 2013]; 6) CO, CO2 and 
CH4 [e.g., Del Bianco et al. 2013]; 7) Minor 
Trace Gases [e.g., Papandrea et al. 2013]; 8) 
Clouds, Aerosols and Water Vapour; 9) Meth-
ods and Instruments for Thermodynamic 
Variables. Scientists and Cal/Val experts have 
been provided with the opportunity to present 
latest results from their on-going research, in-
cluding the evolution of algorithms and new 
validation methods, not restricted to ESA mis-
sions. 125 participants from 14 countries were 
registered with the large majority from ESA 
member states and a significant participation 
from the USA and Canada. 
A set of recommendations for the evolution of 
the ESA’s activities, resulting from the work-
shop discussions, are reported in the next 
paragraphs. The recommendations have been 
grouped in the following seven categories: 
Calibration and Level-1 products, Spectros-

copy, Product Development/Evolution, Algor-
ithms and Products Review Process, Valida-
tion Infrastructure, Data Utilisation, and Evo-
lution of Validation Needs. ESA will carefully 
assess and prioritise the ACVE2013 recom-
mendations, noting that a number of recom-
mendations are already being addressed or are 
planned to be considered in the near future. 
In the next paragraphs, recommendations are 
identified with the ‘[RecX]’ label, where X is an 
identification number with no reference to any 
priority ranking. 
Calibration and Level-1 
Important considerations on pre-flight activi-
ties have been raised. Firstly, it was recom-
mended to focus during the development 
phase on intensive instrument on-ground cali-
bration activities, along with the provision of 
the related detailed documentation and the 
maintenance of the instrument models [Rec1]. 
Secondly, it was stressed that the end-to-end 
performance tests of the complete data pro-
cessing system should be accomplished well 
before the satellite launch [Rec2]. 
Spectroscopy 
Spectroscopy plays a fundamental role in the 
atmospheric remote sensing and the necessity 
of establishing a consistent spectral database is 
evident [Rec3]. In particular, there is a strong 
need for support to the development of a ho-
mogenous Ozone cross-section dataset across 
wavelength ranges [Rec4]. In addition to 
Ozone, supporting new spectroscopy activities 
on water vapour and methane in the SWIR 
(Short Wave Infra-Red) range, impacting the 
CO retrievals around 2.3 μm, have been rec-
ommended [Rec5]. 
Product Development and Evolution 
Establishing a program for new satellite pro-
ducts is needed, if possible coordinated among 
ESA, the European Commission (EC) and the 
national space/funding agencies [Rec6]. ESA 
should continue to coordinate the product de-
velopments of Sentinel-5p (S5p), Sentinel-4 
(S4) and Sentinel-5 (S5) [Rec7], taking into ac-
count the individual performances of the dif-
ferent instruments. For future developments, a 
special focus needs to be on the absorption 



ANNALS OF GEOPHYSICS, 56, Fast Track 1, 2013; 10.4401/ag-6535 
 

cross-sections used in the retrievals for the in-
terpretation of differences between products 
[Rec8] and multi-missions calibration aspects.  
As follow on activity to the ACVE, ESA organ-
ised an inter-instrument workshop the “EO 
Level-1 lessons learned” in June 2013 covering 
degradation and pointing aspects. A specific 
recommendation from this workshop is to cre-
ate a Level-1 Quality Working Group (QWG) 
across instrument families to enhance the in-
formation and analysis exchange. The neces-
sity to create a priority list for the Level-1b 
evolution tasks, as identified by the instrument 
QWGs, as well as to make it available to the 
science community for comments and sugges-
tions [Rec9] was recognised. The need to estab-
lish new mission performance criteria for exist-
ing products (e.g. ENVISAT, ERS-2) has been 
identified. These new criteria should be super-
seding pre-launch requirements, based on ev-
olving user requirements and scientific devel-
opments, and should be used for prioritizing 
evolution activities [Rec10]. This could be 
achieved by redefining the performance cri-
teria for products through the revision of exist-
ing requirements as a joint effort from ESA, 
QWGs and the user communities. In order to 
optimise the verification and validation pro-
cess of new Envisat/ERS product releases, 
well defined verification and validation 
datasets need to be provided to the Cal/Val 
community [Rec11]. In addition to the QWGs, 
it should also involve teams outside ESA 
funded projects, i.e. through national and 
international activities. This activity would fa-
cilitate the establishment of a joint cycle for re-
processing and re-analysis within the atmos-
pheric composition community [Rec12]. 
Algorithms and Products Review Process 
The ACVE2013 community recommends the 
establishment of a review process for evolution 
new algorithms and new mission activities 
[Rec13]. 
Validation Infrastructure 
An essential aspect for future ESA Cal/Val ac-
tivities consists in securing the existing 
Cal/Val infrastructure and establishing an en-
tity taking over the ownership and the respon-

sibility [Rec14]. In parallel, new developments 
in validation infrastructure should be sup-
ported. In particular, the improvement of air-
borne MAX-DOAS systems [Rec15], the cre-
ation of Total Carbon Column Observing Net-
work (TCCON) [Toon et al. 2009] sites charac-
terised by different illumination/albedo condi-
tions [Rec16]. The use of mini-spectrometer 
instruments in the Network for the Detection 
of Atmospheric Composition Change 
(NDAAC) [Rec17] shall be facilitated by pro-
viding support to the development and 
operation of instruments targeting Air Quality 
(AQ). Further investigating options to support 
"mini-interferometer" developments and oper-
ations for high-precision greenhouse gas 
measurements were addressed. This should be 
done in conjunction with the implementation 
of systems supporting the validation of minor 
trace gases (e.g. HCHO, BrO, OClO) and of 
tropospheric Ozone [Rec18]. The identification 
of reference validation sites for both past and 
future ESA missions [Rec19] should be ad-
dressed by revisiting the reference site concept 
with CEOS and CVI for atmospheric compo-
sition and specifically air quality (AQ) valida-
tion. In addition to AQ instrumentation, the 
support to stratospheric balloon “anchor” 
measurements in the post-limb sounding era is 
essential in order to address the Montreal pro-
tocol to understand the trends in the strato-
sphere, as well as for the assessment of the re-
maining limb observation systems [Rec20]. 
ESA cooperation and coordination with 
national environmental agencies and the Euro-
pean Environment Agency (EEA, 
http://www.eea.europa.eu/) for satellite vali-
dation [Rec21] shall be undertaken. The need 
to establish a legal framework for air quality 
monitoring that goes beyond the current regu-
lations was also highlighted. This would re-
quire more sophisticated measurement proto-
cols and instrumentation that can also be used 
for satellite validation [Rec22]. A web portal 
covering ESA and non-ESA atmospheric vali-
dation activities, offering links to Cal/Val in-
formation and datasets, and allowing for the 
direct involvement of the international scien-



ANNALS OF GEOPHYSICS, 56, Fast Track 1, 2013; 10.4401/ag-6535 
 

tific community should be implemented, es-
tablishing links with non-ESA Cal/Val teams, 
in particular regarding infrastructure [Rec23]. 
Evolution of Validation Needs 
A clear need to develop validation activities 
for AQ products in "challenging" conditions 
has been identified during the ACVE2013 to be 
documented in an “Air Quality Validation 
Plan”. In particular, areas with strong pollu-
tion gradients, different cloud conditions, vari-
able terrain types and slopes, and high aerosol 
loads should be considered for the installation 
of validation tools [Rec24] [see e.g. Richter et 
al. 2013]. In addition, the validation of back-
ground profile information, used in nadir re-
trievals and relative to the challenging condi-
tions indicated above, is also required [Rec25]. 
As a result the need to establish ground based 
instrument networks in polluted areas [Rec26] 
was highlighted. It is recommended that ESA 
facilitates and co-organises validation cam-
paigns with a focus on AQ satellite product 
challenges. Support for the development and 
operation of instrument developments target-
ing AQ (e.g., mini-spectrometers [Tzortziou et 
al. 2012], AirCORE [Karion et al. 2010]) and the 
implementation of a European mini-
spectrometer/MAXDOAS network is needed. 
The necessity for the improvement of an effec-
tive characterisation of observation errors for 
satellite and correlative validation datasets has 
been identified [Rec28]. 
Data Utilisation 
Satellite data distribution is the core activity 
for a Space Agency, and improvements in this 
context are fundamental for the success of any 
satellite mission. The ACVE2013 community 
recommends to establish a “data product 
utilization advisory group” that should pro-
vide ESA with feedback and advise on data 
accessibility and on prioritisation of develop-
ments [Rec29]. A pressing need highlighted 
during the workshop is the harmonisation of 
the data format for all specific instruments, 
possibly adopting an established, self-
describing, easy readable format [Rec30]. This 
could be achieved by providing all operational 
ESA atmospheric composition products in 

community accepted formats such as netCDF-4 
with the netCDF Climate and Forecast (CF) Meta-
data Convention (http://cf-pcmdi.llnl. gov/). 
On the data dissemination, a coordinated ac-
cess to ESA, European and national data cen-
tres is urgently needed [Rec31]. Generally 
users need to have easy access to data, avoid-
ing complicated registration procedures. 
[Rec32]. 
Conclusions 
The ACVE2013 provided an important and 
valuable forum for Cal/Val and algorithm sci-
entists to present and discuss their results with 
the atmospheric composition satellite remote 
sensing community. In addition, it gave the 
opportunity to highlight and discuss a broad 
range of issues related to the satellite missions 
directly with ESA. On the ESA side, the intense 
exchange with the expert users is of highest 
priority to identify problems and to find op-
portunities to resolve them with the recom-
mendations by the community, as well as to 
prepare future developments. 
ESA will carefully assess and prioritise the 
recommendations and defining activities ac-
cordingly. A number of recommendations are 
already being addressed, e.g., first steps to a 
Level 1b Working Group have been imple-
mented, the existing CEOS Cal/Val portal is 
being extended to cover atmospheric compo-
sition and a number of system developments 
are on-going or under preparation. 
The mission end-to-end data quality im-
provement and evaluation loop is a process 
open to the community. Several mechanisms 
are already in place, e.g., the ESA Helpdesk, 
QWGs and workshops. In the near future, ESA 
is committed to facilitate direct contacts be-
tween Cal/Val entities, through regular ACVE 
workshops in the coming years. 
 
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    /CourierNewPSMT
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    /TimesNewRomanPS-BoldMT
    /TimesNewRomanPS-ItalicMT
    /TimesNewRomanPSMT
    /Trebuchet-BoldItalic
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  >>
  /ColorImageDict <<
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  >>
  /JPEG2000ColorACSImageDict <<
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  >>
  /JPEG2000ColorImageDict <<
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  >>
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  >>
  /GrayImageDict <<
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  >>
  /JPEG2000GrayACSImageDict <<
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  >>
  /JPEG2000GrayImageDict <<
    /TileWidth 256
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  >>
  /AntiAliasMonoImages false
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  /EncodeMonoImages true
  /MonoImageFilter /CCITTFaxEncode
  /MonoImageDict <<
    /K -1
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  /AllowPSXObjects false
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  >>
>> setdistillerparams
<<
  /HWResolution [2400 2400]
  /PageSize [595.000 842.000]
>> setpagedevice